Final answer:
The best approximation of alveolar ventilation is provided by measuring expired CO2, as this correlates with the effectiveness of ventilation and gas exchange in the alveoli.
Step-by-step explanation:
The best approximation of alveolar ventilation is D) Measuring Expired CO2. Alveolar ventilation refers to the movement of air into and out of the alveoli, the tiny air sacs in the lungs where gas exchange occurs. It is an essential aspect of respiration as it ensures the removal of carbon dioxide from the alveoli and prevents its buildup, which could lead to respiratory acidosis. The measurement of expired CO2 provides a direct estimate of how much carbon dioxide is being expelled, which correlates with alveolar ventilation efficacy. During the process of external respiration, partial pressure differences in oxygen and carbon dioxide between the alveoli and the blood in the pulmonary capillaries drive gas exchange. Tools like the Douglas bag method allow for the collection of expired air, enabling the calculation of oxygen consumption and CO2 production during different levels of activity.